The present invention relates to an electrochromic display device and also to a method of producing it.
In general, an electrochromic display device (hereinafter termed as ECD) is a non-light emitting type display device relying upon electrochromism (hereinafter termed as EC) wherein an optical absorption spectrum changes reversibly and causes coloration or bleaching when an electric field or current is applied. The characteristic of the ECD is to decrease eye strain due to it being of a non-light emitting type display as compared with the light emitting type displays such as those provided by electroluminescence display devices, plasma display devices, and CRTs. In addition, the ECD has excellent properties such as non-dependence upon the viewing angle, multicoloration of the device itself and a possible enlargement of areas as compared with other non-light emitting type display devices such as liquid crystal display devices.
Transition metal oxides have often been used as EC substances which cause colouring or bleaching, together with an oxidation or reduction reaction by injection or ejection of ions and electrons by application of voltage. Thus, a known ECD comprises a display electrode and an opposite electrode which together sandwich one, two or more than two EC substance layers and an electrolyte layer which provides ions to the EC substance layer.
An earlier disclosure, pertaining to vacuum deposition, is found in JP-A-60-238818, in which tungsten oxide is used as an EC substance and an inert gas, namely argon, is introduced as the gas providing the atmosphere in which reaction is to take place. Such conventional processes are carried out at high temperatures (in the region of 280.degree. C.) and very low vacuum ( &lt;10.sup.31 5 torr)
Attempts have been made to use an ECD as a writable display device capable of illustrating letters or figures by means of a surface writing electrode.
The term "write" used herein means "cause a colouring reaction in a specified area to be coloured". In addition, the term "writable" refers to a state where the coloured portion may be sufficiently identified without spreading or narrowing which causes unclearness, and the written letters or figures are clearly identified. Conventional displays such as a liquid crystal display and CRTs have been developed as writable devices of the surface writing electrode type provided with a location detecting function by electrostatic induction.
The following writable ECDs have been proposed. A method is found in U.S. reissued Pat. No. 28,199, in which writing can be performed by contacting an exposed EC layer directly with a writing electrode. Methods are also proposed in JP-A-255920, JP-A-62-267727, JP-A-62-267728 and JP-A-62-286024, wherein at least one of an electrode, EC substance layer and an electrolyte layer is divided into minute, discrete, "pixels", which prevent spreading of the coloured area, thereby enabling ECD to be writable. A method is employed in JP-A-62-156720, in which the ECD may be writable by application of pressure upon such units with a pen, coloration being produced by applying, in advance, a voltage less than a coloration threshold, followed by applying a voltage which exceeds the threshold as a result of the pressure of the pen.
There are, however, drawbacks in each of the above drawn devices. For example, the method disclosed in U.S. reissued Pat. No. 28,199 in which the exposed EC layer is contacted with the writing electrode has defects in that the coloured area overspreads and becomes difficult to read, and the EC layer degrades severely and may not withstand repeated use. On the other hand, as an alternative embodiment, also disclosed in this document, in which the entire EC layer is covered by a conductive layer, the application of a writing electrode to this causes coloration of the entire EC layer.
The method disclosed in JP-A-62-255920- required a complicated patterning process of minute pixels. The method disclosed in JP-A-62-156720 has a defect in that the configuration and circuits of the ECD cell are complicated and it is difficult to use a solid electrolyte. The method disclosed in JP-A-60-238818 is not writable.